1. Field of the Invention
The present invention broadly relates to Multiple Access communication systems. More particularly, the present invention relates to improvements in the temporal alignment of the components in Multiple Access systems.
2. Brief Description of Related Developments
Communication systems such as those involving cellular tower base stations at their center often must share limited power and bandwidth resources with a large number of users. To provide large-volume, multiple and contemporaneous access to bi-directional communication links between Access Points (APEs) associated with the base station and Customer Premises Equipment (CPE) such as mobile telephones, several access schemes have been used. In a Frequency Division Multiple Access (FDMA) scheme, users are assigned a unique communication frequency band that they may use to communicate at any time. The segregation of users according to frequency bands is not without problems including a requirement that transmitter amplifiers must often be operated in a “backed off” condition to avoid unwanted intermodulation products, as will be appreciated by those skilled in the art to which the present invention pertains.
To address the limitations FDMA, other access schemes have been used, including Time Division Multiple Access (TDMA) and Code Division Multiple Access (CDMA). In a TDMA scheme, users share a common communication frequency band; however, each user is assigned a unique time slot for communication. In a CDMA scheme, the users can communicate over the same frequency band at the same time. To distinguish communications, each carrier is modulated with a unique pseudo random code, such as by means of a direct sequence or by frequency hopping spread spectrum modulation.
Other access schemes and potential combinations of those schemes will be familiar to those skilled in the art, including, inter alia, Random Access (RA) and Demand Assigned Multiple Access (DAMA).
In multi-sectored communication systems requiring the synchronization of base station equipment (such as with synchronous CDMA schemes), there is a practical requirement that all sector equipment be synchronized to within a very small fraction of the highest expected sampling rate for proper functioning (i.e., there must be a low skew between the clocks). Because of system design requirements and eccentricities, the sector transmitters are not always equidistantly located from the master clock source. For example, in some geographic locations where a traditional communications tower is not available, the system components might be located in and around a building structure. The difference in transmission lengths causes the master clock signals to arrive out of phase at the various transmitters (relative to each other), thus leaving them in a state of asynchrony.
The three oft most approaches of the prior art to obtain clock signal synchrony at the sector transmitter equipment are 1) redesign the system so that the transmitters are equidistant from the master clock circuitry, which is not always practical, 2) match the cable lengths between each transmitter and master clock circuit, or 3) add delay matching circuitry. Matching the cable lengths may not only involve wasted resources where the differences are large, but also requires interface circuitry. Typical delay-matching circuitry is often unstable with respect to operating temperature ranges, and adds additional cost to the system.
Therefore, what is needed but non-existent in the prior art, is an approach for synchronizing transmitter clocks that are multi-distantly located from the master clock circuitry, that does not entail the additional complexity and cost of introducing interface and complicated delay matching circuits, nor entails the impractical requirement that all transmitters be located the same distance from the master clock circuit.